Commit Message

From: Brian Foster <bfoster@redhat.com>
commit 59e4293149106fb92530f8e56fa3992d8548c5e6 upstream.
Page writeback indirectly handles shared extents via the existence
of overlapping COW fork blocks. If COW fork blocks exist, writeback
always performs the associated copy-on-write regardless if the
underlying blocks are actually shared. If the blocks are shared,
then overlapping COW fork blocks must always exist.
fstests shared/010 reproduces a case where a buffered write occurs
over a shared block without performing the requisite COW fork
reservation. This ultimately causes writeback to the shared extent
and data corruption that is detected across md5 checks of the
filesystem across a mount cycle.
The problem occurs when a buffered write lands over a shared extent
that crosses an extent size hint boundary and that also happens to
have a partial COW reservation that doesn't cover the start and end
blocks of the data fork extent.
For example, a buffered write occurs across the file offset (in FSB
units) range of [29, 57]. A shared extent exists at blocks [29, 35]
and COW reservation already exists at blocks [32, 34]. After
accommodating a COW extent size hint of 32 blocks and the existing
reservation at offset 32, xfs_reflink_reserve_cow() allocates 32
blocks of reservation at offset 0 and returns with COW reservation
across the range of [0, 34]. The associated data fork extent is
still [29, 35], however, which isn't fully covered by the COW
reservation.
This leads to a buffered write at file offset 35 over a shared
extent without associated COW reservation. Writeback eventually
kicks in, performs an overwrite of the underlying shared block and
causes the associated data corruption.
Update xfs_reflink_reserve_cow() to accommodate the fact that a
delalloc allocation request may not fully cover the extent in the
data fork. Trim the data fork extent appropriately, just as is done
for shared extent boundaries and/or existing COW reservations that
happen to overlap the start of the data fork extent. This prevents
shared/010 failures due to data corruption on reflink enabled
filesystems.
Signed-off-by: Brian Foster <bfoster@redhat.com>Reviewed-by: Christoph Hellwig <hch@lst.de>Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>
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fs/xfs/xfs_reflink.c | 1 +
1 file changed, 1 insertion(+)